Hair all goes weird once I change the mesh
It's easy before going into edit mode go to particle system disconnect all hair system by clicking on "disconnect hair"
Edit your mesh and reconnect hair
NOTE: - if you have activated children of hair then first connect the hair and re-activate children 
To re-activate Children: - go to children tab in particle group and click on "none" Then click on 'simle' or 'interpolated' by your choice.  

Hope it's helpful to you.